Gunshot Residue Analysis

Question 1

What is gunshot residue?

Answer 1

Any particles or residues discharged from a firearm after the trigger has been pulled. These may include chemicals from the primer, propellant, oxidisers, reducing agents, sensitisers and binders

Question 2

Sources of GSR

Answer 2

• Residues formed through the explosive reaction of the primer compounds
• Materials from the bullet and bullet jacket
• Materials eroded from the cartridge case, primer cup and other cartidridge components
• Materials from the interior of the firearm chamber and barrel

Question 3

How is GSR formed?

Answer 3

• Formed as a result of rapid cooling of the discharge gases and solid matter
• Some of the gases condense in the form of spheres, but they also interact with soild residue materials to form complex mixtures and aggregate forms
• Most residues will show evidence to or formation at extremely high temps and pressure

Question 4

Collecting GSR from humans

Facts about it not the process

Answer 4

• The subject’s use of their hands should be minimised
• Medical practitioners can swab nasal passages with the subjects permission (there is no law that this can be taken by force)
• All biological GSR samples must be refrigerated immediately
• Hand sampling areas (separate stubs should be used for each hand): back of each hand and the palm of each hand

Question 5

Process of taking GSR samples from suspects hands

Answer 5

• Dab the hand sampling areas in a ‘line search pattern’ up and down the palm, fingers and webbing with an SEM stub in its holder
• Separate stubs for at least front and back of hand

Question 6

Anti-contamination procedures

Answer 6

• Avoid dealing with other items or evidence that can be contaminated before sampling for GSR
• Wash hands and wear PPE (minimum requirement is gloves)
• Take the samples as soon as to avoid loss of evidence

Question 7

GSR: close range

Answer 7

• GSR on surfaces other than human tissue is best recovered by removing as large a section as possible
• The overall residue pattern can be just as important as the chemical composition of the particles
• Dyes can be used to stain the GSR for pattern analysis
• Any samples for chemcial analysis should be taken before staining

Question 8

GSR: long range

Answer 8

• GSR will not be present much beyond a few meters from point of shot
• Important residues will be present on the bullet and will deposit onto surfaces it interacts with - leaves a wipe ring if it passes through the impacted target
• Collection process should ideally be conducted in a lab

Question 9

Greiss test

Answer 9

• A presumptive chemical test which suggests the presence of organic nitrite compounds
• It is used to test for traces of explosive materials and propellants and turns brown/orange in their presence
• It can give false positive test

Question 10

Sodium rhodizonate test

Answer 10

• Chemical metal deposition test which suggests the presence of lead
• Produces a red/pink stain suggesting the possibility of lead
• The area is then treated with dilute HCl and if the stain turns blue then the presence of lead is confirmed
• A positive result from this test and the Greiss test is strong evidence that a firearm has been discharged

Question 11

SEM-EDX

Answer 11

1. Electron gun fires a beam of electrons at high voltage through a vacuum
2. The object will then scatter the electrons by different amounts depending on its orientation and composition
3. The scattered electrons hit a detector and are recorded
4. This then builds a pictures of what the object we are studying looks like
   * Non-destructive technique
   * Minimal sample prep needed
   * Individual particles can be analysed
   * Highly effective and fairly rapid automated systems

Question 12

SEM-EDX partical searching

Answer 12

• Can be applied either manually or in automated mode
• In both modes the search targets particles of high mean atomic number
• These are visualised using a Back Scattered Electron (BSE) detector
• Once a particle with a bright BSE image is detected, it is analysed by EDX

Question 13

Classification of GSR particles

Answer 13

1. Characteristic of GSR - has compositions that are rarely found in particles from any other source
2. Consistent with GSR - has compositions that are also found in particles from a number of relatively common, non-firearm sources
3. Commonly associated with GSR - has compositions that are also commonly found in environmental partiles from numberous sources, but when found in addition to particles that are characteristic of, and/or consistent with GSR these particles can be of significance

Question 14

What does ‘major’, ‘minor’ and ‘trace’ mean on EDX spectrums?

Answer 14

• Major = has a peak height greater than 30% of the highest peak
• Minor = has a peak height between 10% and 30% of the highest peak
• Trace = has a peak height less than 10% of the highest peak
  These thresholds are usually based on the relative peak sizes for the L-transitions for Barium and any values should be greater than the limit of detection for the instrument to be considered

Question 15

Characteristic of GSR

What must it contain (only for sinoxid)

Answer 15

• Lead
• Antimony
• Barium

Question 16

False positives of GSR

Examples of where they can happen

Answer 16

• Car air bags - have a primer device in them and so the residue particles obtained from them after deployment can contain things that are in GSR
• Firewroks & pyrotechnics - uncommon firework products (crackering ball) can contain lead, antimony and barium
• Car brake pads - can contain lead, antimony and barium but thats not as common now, levels of iron (and other elements) are not ususally seen in GSR compared to car brake pads